Elucidating effect of acid strength on isomerization mechanisms of butene over solid acid catalysts in C4 alkylation

Abstract

Understanding the effect of acid strength on butene isomerization mechanisms is essential in C4 alkylation process. Herein, the double bond, trans-cis, and skeleton isomerization mechanisms of butene over acid sites with different acid strengths were investigated utilizing density functional theory (DFT) calculations. It is demonstrated that the acid strength significantly affects the isomerization mechanisms of butene. The stability of carbonium ions as reaction intermediates follows the order: tert-butyl cation > cis-butyl cation > trans-butyl cation, which is more sensitive to acid strength than that of butoxide species. Therefore, the reaction pathways of double bond and trans-cis isomerizations changed with varying the acid strength. Based on the analysis of activation energy barriers, it is found that 1-butene and 2-butene can be easily converted into each other over strong acid site. Thus, for reactants, C4 alkylation has no selectivity to 1-butene and 2-butene over strong acid site, which is consistent with experimental study. This work provides mechanistic insights into the relationship among acid strength, type of butene, and quality of alkylate in the C4 alkylation process.